The earliest hybrid toxins (constructed with peptide hormones) were unable to demonstrate toxicity. Later hybrids demonstrated specific cytotoxicity, but performed badly; i.e., the hybrid toxicity was less effective than the parent toxin. More recently, however, several hybrids have shown both a high degree of selectivity and potency against tumor cells. A variety of tests indicated that entry of "A" chain subunits of ricin into the cytosol compartment was facilitated by either certain classes of receptors or by certain toxin binding chains. Later tests showed that receptors which were used to internalize lysosomal hydrolyses (hydrolysis catalyst found in lysosomes) into the lysosomal compartment, mannose-6-phosphate receptors, could also efficiently internalize the hybrid mannose-6-phosphate-ricin into the cytosol compartment and inactivate protein synthesis. Excess lactose in the medium was used to block the usual ricin entry route via its receptor. Experiments then showed that a binding site on the ricin B chain was necessary for the efficient entry of mannose-6-phosphate ricin into the cytosol compartment. This invention teaches new ricin hybrids with a binding chain consisting of a monoclonal antibody with an affinity for the murine thymic adult differentiation antigen, Thy 1.1 or Thy 1.2. The monoclonal antibody is thioether linked to an intact ricin.